Magnetoresistive or MR heads are operated by sensing the change in resistivity (measured by changes in voltage) across a given area through which a bias current having a constant value is present.
Because of the DC voltage present across the MR head and because of the extreme sensitivity of the head to noise introduced to it through its bias current source, it has been found to be necessary to use at least one DC blocking capacitor per MR head circuit. (At least where an integrated circuit was used to feed the bias current to each MR head using the bypass capacitor. Otherwise a bias source capacitor was also required for each MR head.)
A single integrated circuit may be designed to provide all the support circuitry for all the MR heads on a data storage device arm. Where this is for four MR heads, at least five external (off-chip) capacitors or as many as twelve would be required. This would also require between nine and twelve extra chip leads to support the external capacitors. Capacitors of the small but appropriate size required are expensive and difficult to install in this type of hybrid circuit environment. It is expecially problematic because of reliability problems contributed by the added number of discrete element which would be assembled into the disk drive's head arm assembly. Increased contamination may also be a problem with the more numerous discrete components.
External capacitors are a requirement because the capacitance available on-chip for a read chip capacitor may only extend as high as 20 pf with today's technology. What is needed to accomplish the functions described is a capacitance on the order of 100,000 pf (or 0.1 micro farrads). The typical MR head responds to changes in ambient flux (which indicate data in the adjacent platter) with changes in voltage on the order of 0.2 millivolts. Clearly at these levels of sensitivity Johnson noise and shot noise (recombination noise) are going to be severe problems without the use of the off-chip capacitors. As described above, the prior art would isolate each MR head with at least one off-chip capacitor.
By multiplexing a single outside capacitor for all circuit bias current sources and a single capacitor for all head amplifiers, the total external component count is reduced to two. This does, unfortunately, result in the limitation that only one of the many heads which must use the off-chip capacitor can operate at one time. (This limitation may be overcome by wiring the chip with a plurality of capacitors or by using a plurality of chips.) With this invention, therefore, most of the read/write circuitry on the arm is switched off when not in use.